The present invention relates to pressure sensitive adhesives and more particularly to acrylic pressure sensitive adhesives that possess enhanced adhesion to low energy substrates.
Acrylic pressure sensitive adhesives have been gaining commercial importance over the last 30 years. Due primarily to their polar nature, it is known that acrylic pressure-sensitive adhesives (PSA's) do not have good adhesion to low surface energy substrates, such as typified by polyolefins. Typically, a 180.degree. peel on untreated polyethylene, for example, of less than 0.5 lb/in has been observed using commercial acrylic PSA's.
A variety of modifications of acrylic PSA's have been found in the literature with such modifications providing varying degrees of improvement in adhesion to polyolefin and other low surface energy substrates. For example, U.S. Pat. No. 4,243,789 proposes a liquid hydroxyl-containing alkyl acrylate copolymer which includes a major proportion of alkyl acrylates of acrylic acid with smaller amounts of N-vinyl-2-pyrrolidone and a hydroxyl-containing monomer. U.S. Pat. No. 4,728,684 proposes an adhesive which includes a thermoplastic polyester urethane rubber which has been grafted with an acrylic monomer, a chlorinated rubber, and an isocyanate cross-linking agent. U.S. Pat. No. 4,726,982 proposes PSA's which include an acrylic copolymer which is made from monomers which include N-vinyl-lactam, and a tackifier resin. Japanese 63 57 684 proposes adhesives prepared from tertiary amine group-containing copolymers such as, for example, dimethyl aminoethyl methacrylate/2-ethylhexyl acrylate copolymer and bisphenol-A/epichlorohydrin copolymer dispersed in organic solvent. Such adhesive is stated to have good adhesion to polyolefin substrates.
U.S. Pat. No. 5,079,047 proposes a solventless, photoinitiated adhesive comprising, by weight, 60-95% of an alkyl acrylate, 5-40% of a copolymerizable monomer such as acrylic acid, and 10-30% ethylene vinylacetate. U.S. Pat. No. 3,737,483 proposes a carboxylated polymer product comprising maleic anhydride polymerized with an .alpha.-olefin in contact with an ethylene vinyl acetate (EVA) copolymer in the presence of an organic peroxide and organic diluent. U.S. Pat. No. 3,749,756 proposes the same carboxylated polymer 5product sans peroxide and organic diluent. U.S. Pat. No. 4,347,341 proposes ethylene graft copolymers containing anhydride or carboxyl groups which are made from vinyl esters of monocarboxylic acid, maleic anhydride and esters thereof which are radically polymerized in the presence of 30-95% by weight of ethylene homopolymers or ethylene vinyl ester copolymers. U.S. Pat. No. 4,762,882 proposes modified polyolefin resins which consist essentially of a copolymer of ethylene and .alpha.-olefin and an unsaturated carboxylic acid grafted on the ethylene copolymer. U.S. Pat. No. 4,810,755 proposes an adhesive composition comprising a metal-containing composition consisting of an ethylene-(meth)acrylate copolymer grafted with an ethylenic unsaturated carboxylic acid or its acid anhydride and an ethylenic unsaturated carboxylic or its acid anhydride of a metal hydroxide. U.S. Pat. No. 4,908,411 proposes modified ethylenic random copolymers derived from ethylene-.alpha.-olefin copolymers grafted with unsaturated carboxylic acids, styrene-type hydrocarbons, or unsaturated silanes. U.S. Pat. No. 4,917,734 proposes ethylene copolymers which have been grafted with styrene-based, vinyl, acrylic, and/or methacrylic grafting monomers. U.S. Pat. No. 4,977,212 proposes resin compositions comprising a metal-containing composition consisting of an ethylene ester copolymer and an unsaturated carboxylic acid or its acid anhydride, a saponified EVA copolymer, and a hydrophobic thermoplastic resin.
Despite such proposals, there yet is a need in the art for acrylic-based PSA's that possess enhanced adhesion to low surface energy substrates.